In conventional communication methods, such as TDMA and CDMA, the link between a mobile unit and a base unit, is frame based. The base conventionally includes a very precise and relatively expensive crystal clock which is used to time its operations and synchronize every unit therein. A mobile unit which communicates with this base often includes a low quality crystal clock which is of lower precision than the crystal clock of the base unit.
Therefore, the mobile unit includes correction mechanisms which are used to synchronize the frequency produced by the mobile unit crystal to the frequency produced by the base unit crystal.
Reference is made now to FIG. 1 which is a schematic illustration of a mobile unit and a base unit known in the art. Base unit 2 includes a data interface 14, an FEC unit 16 connected to the data interface 14, a transmitter 18 connected to the FEC unit 16 and an antenna 20 connected to the transmitter 18.
The base unit 2 also includes clock mechanism which includes a crystal 13 for generating a basic high frequency and a PLL unit 15 connected thereto. PLL unit 15 is further connected to the transmitter 18 and provides it with a frequency which is m/n of the basic frequency. It will be appreciated by those skilled in the art, that the values set for m and n can be any natural number (including the number 1).
Mobile unit 4 includes an antenna 30, a channel sampler 26 connected to antenna 30, a demodulator 25 connected to the channel sampler 26, an time tracking unit 28 connected to demodulator 25, a voice decoder 24 connected to the demodulator 25, a digital to analog converter (DAC) 22 connected to voice encoder 24 and a speaker 32 connected to DAC 22.
Mobile unit 4 also includes a timing mechanism which includes a crystal oscillator 23 and 2 PLL units 21 and 25. PLL unit 25 is further connected to channel sampler 26 and provides it with a frequency which is m/n of the basic frequency provided by crystal oscillator 23. PLL unit 21 is further connected to the DAC 22 and provides it with a frequency which is l/k of the basic frequency provided by crystal oscillator 23.
The mobile unit 4 also includes a sampling and transmitting section, including a microphone 42, a voice sampler 34 connected to the microphone 42, a voice encoder 36 connected to the voice sampler 34, a modulator 38 connected to the voice encoder 36 and transmitter DAC 40 connected to the modulator 38.
The timing mechanism of the mobile unit is also connected to the transmitting section so that PLL 21 also times and controls voice sampler 34 and PLL 25 also times and controls the transmitter DAC 40.
Accordingly, transmitter DAC 40 is affected by PLL unit 25 and sampler 34 has to adjust accordingly. The sampler 34 has to provide a block of a predetermined number of samples N.sub.BLOCK, for each frame produced by modulator 38. Since the timing of transmitter DAC 40 is dynamic, wherein its frequency may increase or decrease, the voice sampler 34 may provide more or less than N.sub.BLOCK samples in a block, for each frame.
According to frame based communication standards such as TDMA and CDMA, each frame includes a predetermined number of information bits which may be the information data, voice data and the like. According to TDMA standard, each frame lasts 20 ms. A voice information frame includes 160 voice samples.
Accordingly, the data interface 14 provides blocks to the FEC unit, which include N.sub.BLOCK samples, in each block, every 20 ms. The FEC unit 16 produces a frame which is then provided to transmitter 18. The transmitter 18 transmits this frame via antenna 20.
At the mobile unit 4, the channel sampler 26 detects the transmitted frame via antenna 30 and provides it to demodulator 25. The demodulator 25 analyzes the frame, extracts voice coded data and provides it to voice decoder 24. The voice decoder 24 decodes this data and provides the decoded signal to DAC unit 22. The DAC unit 22 converts the decoded signal into analog signal and provides it to speaker 32 which in turn produces sounds.
In the mobile unit 4, the channel sampler 26 and the DAC 22 have to be synchronized too. Thus, for every frame detected by channel sampler 26, the DAC 22 has to convert N.sub.BLOCK samples. It will be appreciated that the synchronization mechanisms, of both base unit 2 and mobile unit 4, have to be completely synchronized. Accordingly, the channel sampler 26 has to be synchronized with FEC 16 so that each frame produced by FEC 16 will be received as such in channel sampler 26.
Conventional communication systems include a calibration mechanism which constantly calibrates PLL 25 so as to synchronize it according to the transmitting rate of the base unit 2. It will be appreciated that since DAC 22 is dependent on channel sampler 26 changing the frequency on which channel sampler 26 operates will affect DAC 22. For example, if the frequency of channel sampler 26 increases, then the DAC 22 may be provided with more than N.sub.BLOCK samples in 20 ms.
On the other hand, if the frequency of channel sampler 26 is decreased then the DAC 22 might be provided with less than N.sub.BLOCK samples per frame. Both of these situations are not allowed in TDMA and for that matter in most frame base communication standards.
Another method known in the art deletes or estimates the last samples. If the current number of samples which are to be provided for a block exceeds N.sub.BLOCK samples than N.sub.BLOCK samples are selected and provided as a block and the rest of the samples are deleted. If, on the other hand, there are less than N.sub.BLOCK samples and time comes to provide them to the voice encoder then samples are estimated according to the existing samples so as to provide complementary samples which will complete the block to N.sub.BLOCK samples. It will be appreciated that according to this method, the quality of speech is reduced since an additional element of distortion or noise is introduced, which did not exist in the original sampled or the decoded signal.